Preheat the oven to 350º. Place foil on top of one another to form a "t". Place Spinach leaves on top, next layer the top of the spinach with corned beef, then the onion, tomato and dashes of salt and pepper. Top off the layers with the mayonnaise. Wrap the first layer of foil carefully, then wrap the next layer on top. Bake on a baking sheet in a 350º oven for 1 hour. Remove and allow it to cool for at least 10 minutes. Carefully open the package and stir the Lu Pulu. Serve with Kumala (Yams or Sweet Potatoes).
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